1. Field of the Invention
The present invention relates to a plasma processing apparatus which performs fine processing of various kinds of semiconductor devices, like LSIs, and thin film devices using discharged plasma.
2. Description of the Related Art
One of conventional plasma processing apparatuses is a parallel plate type apparatus which causes a high-frequency power supply discharge between two facing electrodes. FIG. 1 shows a conventional plasma processing apparatus described in Unexamined Japanese Patent Publication No. Hei 3-68136. Two facing electrodes 31 and 32 are disposed inside a vacuum container 1. The electrode 31 is provided on the top of a shaft standing upright at the bottom portion of the vacuum container 1 via an insulator 34, with its surface parallel to the shaft's top. The electrode 32 is provided on the bottom end of a shaft suspending upright from the top wall of the vacuum container 1 via an insulator 35, with its surface parallel to the shaft's bottom end. A connecting section of a gas intake pipe 4 is provided at the upper potion of one side wall of the container 1, and a connecting section of a gas exhaust pipe 5 is provided at the bottom wall of the container 1. One of the two electrodes, 32, is grounded, and the other electrode 31 is connected to a high-frequency power supply 6. The pressure inside the container 1 is set to a predetermined level by guiding gas into the container 1 via the gas intake pipe 4, discharging the gas from the container 1 via the gas exhaust pipe 5 and adjusting the gas supply speed and gas discharge speed. A high-frequency voltage is then applied between the electrodes 31 and 32 to generate plasma to perform a plasma process on a to-be-etched substrate 2 which is placed on the electrode 31.
For example, the substrate 2 to be etched is placed on the high-frequency electrode (31) side in the vacuum container 1, and an etching gas 41 is supplied from the other electrode (32) side via the gas intake pipe 4. With the pressure in the vacuum container 1 kept at the vacuum pressure of 0.3 to 1.0 Torr by discharging the gas from the gas exhaust pipe 5, high-frequency power is applied between the electrodes 31 and 32 by the high-frequency power supply 6, thus producing plasma. A negative self-bias voltage is produced on the electrode side to which the high-frequency voltage is applied, and ions accelerated by the negative bias enter the to-be-etched substrate 2 so that anisotropic etching is performed on the substrate 2.
Because the electrodes of this apparatus have a parallel plate type structure, a voltage Vdc can be applied to the to-be-processed substrate 2 uniformly over the surface thereof. This ensures uniform ion energy to be incident to the surface of the to-be-processed substrate, so that uniform plasma processing is accomplished.
Another parallel plate plasma processing apparatus uses the electrode 32 which has a plurality of gas outlet holes 32a as a ground electrode, and is disposed to face the electrode 31 on which the to-be-processed substrate 2 is to be placed, as shown in FIG. 2. As the electrode 32 having the gas outlet holes 32a has a hollow support shaft 40 in which a process gas 41 is supplied so that the process gas is injected into the container 1 from the holes 32a. Therefore, the connection portion for the attachment of the gas intake pipe 4 is not provided on the side wall of the container 1.
The use of the electrode 32 having such gas outlet holes ensure the uniform flow of the gas to be supplied to the to-be-processed substrate 2, thus further improving the uniformness of the plasma processing of the to-be-processed substrate 2. Because of their simple structures as is apparent from FIGS. 1 and 2, the illustrated apparatuses are widely used today.
According to those conventional apparatuses, the collision of radicals with ions bends the incidence of the ions to provide a plasma distribution in the ion incident direction. It therefore becomes difficult to process a fine pattern with the size of a sub micron order. To cope with the difference in size conversion caused by the oblique incidence of ions, it is inevitable to reduce the discharge pressure and align the ion incident directions. This is because that the low pressure increases the means free path of ions, so that the probability of collision of radicals with ions becomes smaller and the ion incident directions are aligned, causing the ions to be incident perpendicularly. In the low pressure area, however, plasma is likely to spread in the chamber, causing unstable discharge and making it difficult to trap the plasma. Naturally, the plasma density becomes lower, so that a sufficient processing speed cannot be obtained.
Recently, a narrow-electrode type parallel plate plasma processing apparatus is also used which traps plasma between the electrodes to improve the plasma density on the object to be processed, thus increasing the plasma processing speed.
The aforementioned Unexamined Japanese Patent Publication No. Hei 3-68136 discloses the method of acquiring high-density plasma even in a low-pressure area by using a hollow electrode. The structure of this apparatus is illustrated in FIG. 3. This apparatus has a ground electrode 32 provided at the upper end of the support shaft which stands upright at the bottom of a vacuum container 1 via an insulator 90. A substrate 2 to be processed is to be placed on this ground electrode 32. Disposed above the ground electrode 32 in parallel thereto is a high-frequency electrode 31. This high-frequency electrode 31 has a center hole 33.
A gas intake pipe 4 is supported, perpendicular along its lengthwise direction, in the top wall of the container via an insulator 93. A hollow electrode 7 is provided at the lower end of the gas intake pipe 4, with its opening facing downward. Attached to the midway of the gas intake pipe 4 via an insulator 91 is a cylindrical anode or positive electrode 8 whose lower end is open and which hangs perpendicularly along its lengthwise direction. One of the aforementioned parallel plate type electrodes, 31, is attached via an insulator 92 to the lower end of the positive electrode 8. The electrodes 31 and 32 are connected via a switch 12 to a high-frequency power supply 6. A negative voltage supply 61 is connected to the gas intake pipe 4, so that a negative voltage is applied to the hollow electrode 7.
As a DC voltage which is negative on the hollow electrode (7) side is applied between the hollow electrode 7 and opposing electrodes 31 and 32 in this conventional apparatus, plasma discharge occurs locally in the hollow electrode 7 into which the etching gas is supplied via the gas intake pipe 4, and the discharge is trapped in the hollow electrode 7 so that high-density plasma is produced. The power supply from the high-frequency power supply 6 also generates plasma between the opposing electrodes 31 and 32. The plasma in the hollow electrode 7 is supplied via the hole 33 between the parallel plate electrodes 31 and 32 to be put over the plasma that has been produced between the parallel plate electrodes 31 and 32. Consequently, high-density plasma can be obtained. This is a triode type method.
The above-described processing apparatuses all have the following shortcoming. The conventional apparatuses, which execute plasma processing using the electrodes whose distance therebetween is shortened, increase the plasma density and enhance the etching speed. But, the etching gas and reaction product are discharged around, so that the density differs between the center portion and the peripheral portion of the object to be processed. In addition, because of the narrow gap between the electrodes, the parallelism of the two electrodes affects the uniformity of the electric field. It therefore difficult to acquire uniform plasma over the to-be-processed object.
Moreover, while the aforementioned triode type RIE or the like which has been proposed to obtain high-density plasma can locally acquire high-density plasma, the plasma is carried over the surface of the to-be-processed object by diffusion so that the substantial plasma density is insufficient over the object's surface.